Vehicle Location With Reed Switches

Initially I discarded the idea of using the magnet on the steering guide arm to operate reed switches as, strong though they are, they can't operate a standard reed switch mounted under the baseboard. As a result I tried experimenting with a second magnet mounted on the vehicle to operate under baseboard reed switches.

To start with I placed a rare earth magnet on the vehicle‘s centre line behind the front axle. This had no effect until the vehicle was stopped using a magnetic stop at which point the magnet on the vehicle kept the reed switch ‘locked’ in the off position, even when the magnetic stop was released.

Placing the magnet at the rear of the vehicle put it too far away from the reed switch mounted underneath the baseboard for reliable operation.

Reading the F-Track specifications provided inspiration. They use a reed switch mounted just under the road surface and positioned in place of the steel guide wire. The magnet follows the reed switch quite happily and all the magnetism is available to operate the switch.

Unfortunately the steering arm magnets are not powerful enough to operate the standard cheap reed switches available in bulk. You need to use more sensitive ones such as the CL38R example available from Maplin Electronics.

To install the switch you need to bend the two leads so that they run at right angles to the switch. Hold the end of the leads next to the switch body with pliers when you do this to avoid breaking the glass tube and ruining the switch. Dig out a channel to fit the switch along the course of the guide wire. The top of the switch should be level with the top of the guide wire.

Now drill two holes so that the leads can run through the baseboard. Glue the switch in position and then run the guide wire up to each end of the leads and fix in place. Now fill in the guide wire and switch trenches and make good the road surface. Finally connect the leads to whatever electrical circuitry you are using.

The magnets used on lorries and buses will work the reed switch without a problem and can be used to flash an LED as the vehicle passes over the switch. Vehicles with smaller magnets, such as the Ford Transit van, do not operate the switch. You will need either to add a second small magnet to the steering arm or replace the magnet with a larger one. (Obviously one large magnet would enable you to convert two vehicles - one using the large magnet, the other using the two small ones.)

After further experimentation I have opted to use Hall Effect switches instead of reed switches.

See also the related topic of vehicle identification.